If you go to the PayPal payment notification you received when they paid and click reply you'll see the buyer's email in the ''to'' box.
If you haven't got that notification any more, just use your own.
The only thing I've ever gotten from couriers who've been given my email purportedly to contact me should ''issues'' arise with my delivery is a confirmation of delivery notification, a load of spam and unrelated promtional junk
